The following relates generally to the optical components used in optical communication networks, and specifically to an optical device that can filter optical signals, with an adjustable bandpass, or filter characteristic.
Optical amplifiers are extensively used in fiber optic networks in order to compensate for the attenuation loss of the optical signal while traveling along fibers and passing through other optical components. Amongst a few types of optical amplifiers, the erbium-doped fiber amplifier is widely deployed in worldwide fiberoptic networks, as a result of its strong optical gain and excellent performance in the operating wavelength range (1530 to 1570 nm) of typical optical networks.
FIG. 1A exemplifies a representative Erbium-Doped Fiber Amplifier (EDFA) of the prior art. The optical signals at various points within FIG. 1A are depicted in FIGS. 1B, 1C, and 1D. An input optical signal 101, having an optical power spectrum as indicated in FIG. 1B, enters an EDFA and is optically boosted by stimulated emission within the erbium-doped fiber to become a stronger output signal 102, having a power spectrum as shown in FIG. 1C. However, Amplified Spontaneous Emission (commonly referred to as ASE) occurs simultaneously with the stimulated emission. This results in an unacceptable level of noise that is present with the intended output signal. Therefore a wavelength filter 103 in FIG. 1A is used to filter out the unwanted ASE noise, as shown in FIG. 1D, before the signal continues to propagate down the fiber, or is sent to a photo-detector or optical receiver. By filtering out much of the ASE noise, the wavelength filter 103 improves the signal-to-noise ratio of the optical signal.
In modern intelligent or re-configurable optical networks, the wavelength of the input optical signal may be changed from time to time. For example, a tunable optical transmitter may be used to generate the input optical signal. Thus, a tunable optical filter 103 may need to respond to the incoming wavelength change. Furthermore, the optical signal power is modulated to carry the desired information. The higher the modulation rate, the wider the bandwidth of the optical signal or wavelength. As an example, the modulation rate of present advanced optical communication systems can go up to 400 Gbits/second. This creates a Full-Width-Half-Maximum (FWHM) wavelength bandwidth of about 1.3 nanometers. In order to prevent distortion of the filtered optical signal, the FWHM bandwidth of the filter is typically 3 to 5 times that of the signal. Thus a tunable optical filter with a FWHM bandwidth of 3 nm to 8 nm is useful.